The periodic backup metrics resync in updateTotalBackupMetric only set
backupLastSuccessfulTimestamp values but never removed stale entries.
When a schedule was deleted and its backups removed, the gauge persisted
until the Velero pod was restarted.
Reset the gauge before re-setting current values so that deleted
schedules are pruned automatically each resync cycle.
Fixes#9239
Signed-off-by: Shubham Pampattiwar <spampatt@redhat.com>